Discontinuous, asymmetric and irregular colour patterns in Silurian oncocerids (Nautiloidea) with cyrtoconic shells.

The colour pattern is a conspicuous character of nautiloid shells. Better understanding to its evolution and possible function is limited by the rarity of preservation. The majority of colour patterns are preserved in demersal late Silurian breviconic oncocerids, while in other cephalopod orders and shell forms, it is limited or unknown. The colour pattern of late Silurian oncocerids (Oonoceratidae Flower, 1942 and Oocerinidae Teichert, 1939) possessing cyrtoconic and slightly curved shells is documented in nine species from Bohemia belonging to five genera. The cyrtocones Richardsonoceras Foerste, 1933 and Oonoceras Hyatt, 1884 exhibit slightly irregular oblique zig-zag patterns, which may be disrupted and may pass into irregular patches of dark pigment. By contrast, the colour pattern in Oocerina Foerste, 1926, possessing similar shell shape to Oonoceras, consists of narrow, transversal, slightly undulated bands. The short and very slightly curved shell of Chromatoceras gen. nov. displays bilaterally asymmetrical irregular bands or, as in the type species, irregular, asymmetrical, strongly discontinuous colouration. Distinct changes of patterning throughout ontogeny are documented herein in oncocerids, including narrowing of bands, irregular juvenile colouration passing into zig-zag bands and ventrally disappearing colouration in adult shell. Bilaterally asymmetric and irregular pattern, for the first time documented in nautiloids, appeared in probably more demersal cephalopods. The asymmetrical pattern exhibits high intraspecific variation unknown in other nautiloids. All studied oncocerids likely inhabited euphotic zone and the primary function of the colour patterns is camouflage. [ABSTRACT FROM AUTHOR]